Date of Award
Doctor of Philosophy (PhD)
The emission of carbon dioxide from the power industry is one of the major causes of climate change. However, it is well established that the development of human societies across the globe depends on reliable and dispatchable power, which are generally the source of these emissions. There has been a recent growth in the integration of intermittent sources such as wind and solar which are carbon free. But these intermittent sources have significant temporal changes in power production and thus lack reliability and dispatchablity. This is a complicated problem to solve. However, flexible power plants with carbon capture and storage, are dispatchable electricity sources that precisely fit the need of the modern grid. One of the most promising carbon capture, utilization, and storage technology for coal power plants is the pressurized oxy-combustion technology. In pressurized oxy-combustion, coal is burned under elevated pressure with oxygen and recycle flue gas to produce a stream consisting mainly of CO2 and H2O. The moisture in the pressurized gas condenses at a higher temperature, which can be integrated to the steam cycle to increase the plant efficiency. Staged Pressurized Oxy-Combustion (SPOC) technology developed at Washington University at St Louis is an advanced version of pressurized oxy-combustion technology. The SPOC process has significantly higher plant efficiency compared to other pressurized combustion technologies because of reduced flue gas recycle. This work is divided into two parts to advance the understanding of pressurized oxy-combustion technology, specifically SPOC technology. The first part of the work focuses on the formation and removal of pollutants and the second part focuses on process development. Coal combustion produces oxides of sulfur and nitrogen (SOx and NOx). These gases are acidic in nature and can lead to pipeline corrosion during the pressurization and transport of CO2. Therefore there is a need to understand the formation of these gases in the combustor and their subsequent removal in a direct contact cooler (DCC). To understand the formation of SOx, NOx, and CO under pressure, we performed experiments in a 100 kWth pilot-scale combustor under a range of excess oxygen concentration and residence times. We found that the formation of SO2 decreases with excess oxygen concentration in flue gas and pressure, potentially because of higher SO3 formation and higher sulfur retention in the ash. The concentration of NO in the flue gas decreased with pressure but increased with excess O2 concentration, mainly because at higher pressure, the fuel nitrogen diffusion out of the char particle is reduced, providing a higher time for reduction to N2. Finally, we also found that the CO concentration in the flue gas decreased at a higher pressure and higher O2 concentrations, suggesting that at higher pressure, combustion can be accomplished with lower excess oxygen, which can reduce the cost of O2 production. The removal of SOx and NOx is performed in a DCC at a temperature below 300° C. The DCC also recovers the latent heat from the moisture making the absorption in DCC high-temperature process. To develop the kinetics of the reaction between absorbed SO2 and NO2, (HSO3– and HNO2) experiments were performed in a CSTR under a varying pH and temperature, relevant to DCC conditions. We found that the reaction rate goes up with increasing temperature and reducing pH. Moreover, lowering the pH also led to the formation of HSO4- in place of a complex called HADS (hydroxyaminodisulfonic acid). A kinetic constant and temperature dependency of the reactions were obtained from the data. Based on these experiments and additional analytical analysis, the overall reaction mechanism in the DCC was reduced and a model of optimal complexity was established. The model consisted of 5 main reactions capable of predicting the kinetics inside the DCC. To understand the transport characteristics and validate the reduced kinetic model, experiments were performed in a pilot-scale DCC. Several parameters of interest, such as pressure, oxygen concentration, inlet gas temperature, NO/SO2 ratio, and liquid to gas ratio, were analyzed. The results suggested that pressure increased the scrubbing of both NO and SO2. However, the impact pressure on SO2 scrubbing increased significantly with increasing the NO to SO2 ratio. We found that increasing the inlet gas temperature had a negative effect on both NO and SO2 scrubbing, but the scrubbing of SO2 through liquid-phase reactions increased with increasing NO/SO2 ratio and temperature. Moreover, we found a significant impact of liquid to gas ratio on SO2 scrubbing but only a mild impact on NO scrubbing. The reduced kinetics was modeled in Aspen Plus and validated against the experimental results, predicting them accurately. Finally, the reduced kinetics was used to model and optimize a full-scale DCC in Aspen Plus. An optimized model with a split water flow design increased the scrubbing efficiency of NO and SO2 by 9% and 3%, respectively, compared to the conventional design. The second section of the thesis focuses on the process development of the pressurized oxy-combustion process. One of the major reasons for efficiency improvement in the SPOC process is the reduced recycle ratio. To understand the impact of recycle on oxy-combustion processes, a fundamental thermodynamic model was developed, which was complemented with a process model in Aspen plus. We found a non-linear impact of recycle ratio on the net plant efficiency of the power plant, with the impact increasing in a hyperbolic mode at a higher recycle ratio. Exergy destruction in the boiler was found to have a more significant impact on the plant efficiency than fan power consumption. Finally, the process design and analysis of modular, pressurized air-combustion (MPAC), carbon-capture ready power plant were performed in Aspen plus to understand the plant efficiency and to provide a pathway to transition the MPAC plant to SPOC power plant. The objective was to develop a power plant that is highly efficient and flexible but can be easily converted to an SPOC power plant when economics allows for such a move. We found that the efficiency of the MPAC power plant was 1.7% higher than conventional air combustion power plants with major components similar to SPOC. A pathway to transition to SPOC was also discussed. This combination of experimental and modeling results and analysis presented in this work hopes to push the development of SPOC process a step further.
Richard L. Axelbaum
Available for download on Tuesday, October 18, 2022